In the decade since the attacks of Sept. 11, 2001, private intelligence firms and security consultants have peeled away veterans from the top reaches of the CIA, hiring scores of longtime officers in large part to gain access to the burgeoning world of intelligence contracting.

At least 91 of the agency’s upper-level managers have left for the private sector in the past 10 years, according to data compiled by The Washington Post.

In many quarters in Washington, government officials decamp for the private sector as a matter of course. Defense consultancies routinely hire generals retiring from the Pentagon; the city’s lobbying firms are stacked with former members of Congress and administration officials.

But the wave of departures from the CIA has marked an end to a decades-old culture of discretion and restraint in which retired officers, by and large, did not join contractors that perform intelligence work for the government.

It has also raised questions about the impact of the losses incurred by the agency. Veteran officers leave with a wealth of institutional knowledge, extensive personal contacts and an understanding of world affairs afforded only to those working at the nation’s preeminent repository of intelligence.

Among the CIA’s losses to the private sector have been top subject-matter experts including Stephen Kappes, who served as the agency’s top spy in Moscow and who helped negotiate Libya’s disarmament in 2003; Henry Crumpton, who was one of the CIA’s first officers in Afghanistan after the Sept. 11 attacks; and Cofer Black, the director of the agency’s counterterrorism center on Sept. 11.

The exodus into the private sector has been driven by an explosion in intelligence contracting. As part of its Top Secret America investigation, The Post estimated that of 854,000 people with top-secret clearances, 265,000 are contractors. Thirty percent of the workforce in the intelligence agencies is made up of contractors.

Those contractors perform a wide range of tasks, among them assessing security risks, analyzing intelligence and providing “risk mitigation” services in foreign countries.

Some of the officials quoted for this report spoke on the condition of anonymity because of the sensitivities involved in discussing the agency’s inner workings.

Few of them cited problems at the agency as their reason for leaving. Rather, they said, the choice was often financially driven.